In what was described as one of the largest criminal copyright cases, the United States accused Megaupload and its founder, Kim Dotcom, of causing more than $500 million in damages to copyright holders through what it said was a criminal enterprise to pirate music, movies and video games.

But in the months since, Mr. Dotcom and his lawyers have succeeded in chipping away at the case. First, a judge ruled that the police had used the wrong kind of search warrant. Then, Mr. Dotcom was returned some of the millions of dollars in assets that had been seized, including a Mercedes-Benz and access to $49,000 each month from his bank accounts. Other critics of the case have raised concerns about the United States government’s decision to use a criminal proceeding for copyright disputes, which are more commonly handled in civil suits.

Nigel Marple/ReutersThe success of Megaupload has allowed Kim Dotcom to own an estate in New Zealand called Dotcom Mansion.

Now a judge in New Zealand has ruled that the United States must share evidence with Mr. Dotcom — including the Federal Bureau of Investigation’s report — while also indicating his own concerns about the case. The United States government had argued that it had to include only a summary of its evidence in the case, but in a ruling issued Tuesday, Judge David J. Harvey of District Court in Auckland said that it had to produce specific evidence to justify the extradition request and to allow Mr. Dotcom to prepare his defense.

“A denial of the provision of information that could enable a proper adversarial hearing in my view would amount to a denial of the opportunity to contest,” Judge Harvey wrote. “That would effectively mean that the process is one-sided.”

The case is being watched closely by media companies, which have long seen Megaupload as a threat; according to the government, Megaupload had 67 million users around the world. Civil liberties groups and others have also been interested in the case, raising concerns about the legality of the seizure.

The charges against Mr. Dotcom and seven other Megaupload employees include criminal copyright infringement, conspiracy, money laundering, racketeering and wire fraud. Mr. Dotcom faces up to 20 years in prison if found guilty.

The judge’s order also raises a question about the nature of the suit. As the judge wrote, the case is criminal, but the basis for the charges against Megaupload are more commonly cited in the civil context of copyright law:

This is a case that is more complex than many. There is a complex factual matrix and the justiciable issues are complicated by the fact that the United States is attempting to utilize concepts from the civil copyright context as a basis for application of criminal copyright liability, which necessitates a consideration of principles such as the dual use of technology or what they be described as significant non infringing uses.

The existence of criminal copyright charges is a keystone to providing the unlawful conduct element of the racketeering, money laundering and wire fraud charges.

When asked on Wednesday whether the United States would appeal the decision, a Justice Department spokesman declined to comment. Mr. Dotcom’s extradition hearing is scheduled for August.